A team of astronomers that includes Dr. Nader Haghighipour of the University of Hawaii at Manoa has discovered the first two-planet system orbiting two stars. The announcement was made at a symposium in August taking place as part of the triennial International Astronomical Union meeting held in Beijing.

Known as Kepler-47 because the data came from the Kepler spacecraft, this planetary system demonstrates that complete planetary systems can exist around a pair of stars.

The system contains the smallest known planet orbiting a pair of binary stars. The radius of the inner planet is only three times that of Earth and orbits the binary stars every 49 days. The radius of the outer planet is 4.6 times that of Earth (about the size of Uranus), and it orbits the binary every 303 days, making it the longest orbital period transiting planet known to date. A planet transits its star when it crosses in front of it as seen from Earth, or in this case, by Kepler.

More important, the outer planet's orbit and the spectral types of the stars (G and M) place the planet well within the "habitable zone," the region where a terrestrial planet could have liquid water on its surface. While this planet is most likely a gas-giant planet like Jupiter and not a terrestrial planet like Earth, and is thus not suitable for life, its discovery establishes that circumbinary planets can, and do, exist in habitable zones.

The stars orbit and eclipse each other every 7.5 days. Each planet transits over the primary star, giving unambiguous evidence that the planets are real. The Kepler-47 system contains at least two planets. There is evidence that at least one additional planet may be present, but the current data are not yet sufficient to establish its existence.

The planets are small, and thus unlike the previous Kepler circumbinary planets, they do not gravitationally disturb the stars or each other, so their masses cannot be directly measured. However, the data clearly show that these small objects are certainly planets and not brown dwarfs (objects larger than planets but smaller than stars). Based on their radii, they probably have masses of about 8 and 20 times that of Earth.

Dr. Jerome Orosz (San Diego State University) is the lead author of the study, which is published in the journal Science. Funding for this work was provided in part by NASA and the National Science Foundation.